Available time slot in a packet switch are identified in order to route a packet from an input port to a designated output port. The basic shape is changed from a cylindrical shape to an elliptical shape, a somewhat rectangular shape, variations on a gabled roof shape, or variations on a gothic arch shape, such that the flow is more uniform and the seeds have less distance to rise to the surface. The gate signal generators are operatively coupled to the carrier signal generators, respectively, to receive the carrier signals, each of the gate signal generators being configured and arranged to generate an onoff signal by comparing a command value of each phase of the polyphase alternating current and corresponding one of the carrier signals. The second web is disposed on an internal side of the second sidewall.